Abstract

We apply coherent anti-Stokes Raman Scattering (CARS) microscopy to characterize director structures in liquid crystals. We demonstrate that the polarized CARS signal in these anisotropic fluids strongly depends on alignment of chemical bonds/molecules with respect to the collinear polarizations of Stokes and pump/probe excitation beams. This dependence allows for the visualization of the bond/molecular orientations via polarized detection of the CARS signal and thus for CARS polarization microscopy of liquid crystal director fields, as we demonstrate using structures in nematic, cholesteric, and smectic liquid crystals. On the other hand, laser-induced director realignment at powers above a well-defined threshold provides the capability for all-optical CARS signal enhancement in liquid crystals. Moreover, since the liquid crystalline alignment can be controlled by electric and magnetic fields, this demonstrates the feasibility of CARS signal modulation by applying external fields to these materials.

Figures (11)

Raman scattering spectra of the studied LC materials. (a) Raman scattering of 8CB for directions parallel and perpendicular to n̂0 in a planar cell; the inset shows chemical structure of the 8CB molecule. (b) Raman spectra for single-compound LC 5CB (red), nematic mixture E7 (green), and a mixture of nematic AMLC-0010 and chiral additive CB-15 (~1 Wt. %). For clarity, the spectra have been displaced with respect to each other along the “Intensity” axis.

CARS-PM setup and the principles of CARS imaging. (a) schematics of the experimental setup: ωs and ωp are frequencies of synchronized picosecond Stokes and pump/probe waves, respectively; M1–M7 are dichroic dielectric mirrors; O1, O2 -objectives; F1 and F2 are filter wheels which allow for selection of CARS-PM signal within 620-820 nm; P1–P6 are polarizers; the XY scanner is based on computer-controlled galvano mirrors; the detectors (photomultiplier tubes) in the E-CARS and F-CARS channels are marked respectively. (b) the energy diagram with the enhanced anti-Stokes Raman scattering signal at was=2wp-ws obtained when wp-ws=wνib.

CARS signal (detected within 620-820 nm spectral range) as a function of the beating frequency ωp-ωs around ωvib of the CN-bond of 8CB; the data are obtained for laser polarizations parallel (red) and perpendicular (black) to n̂0.

Angular dependence of intensity in CARS-PM and its comparison to that in FCPM. (a) Experimental F-CARS intensity vs. the angle θ between P̂CARS and n̂0 in uniformly-aligned planar cell with 5CB; the red line shows the expected ICARS∝cos8θ dependence. (b) Simultaneously-measured F-CARS and E-CARS signals vs. θ obtained in a planar cell of 8CB; the E-CARS signal is weaker than that of F-CARS even at an increased PMT gain. (c) FCPM intensity vs. the angle between P̂FCPM and n̂ in a BTBP-doped nematic LC and the respective theoretical dependence IFCPM (θFCPM)∝cos4θFCPM (red line).

Effects of laser-induced realignment on the CARS-PM signal. (a) CARS intensity vs. the scanning area in a homeotropic 5CB sample. Insets in (a) show the observed laser-induced transition from a uniform state (inset ii) to a distorted state (inset i) observed when the area is decreased or power is increased. (b) CARS intensity vs. the incident laser power Wp+Ws for a constant scanning area 7.5µm×7.5µm and for a diffraction-limited spot of ~1µm2 (inset). (c) CARS intensity vs. WsWp2 for a scanning area of 7.5µm×7.5µm in homeotropic (P̂CARS⊥n̂0) and planar (P̂CARS‖n̂0) cells. (d) CARS intensity vs. WsW2p for a diffraction-limited spot of ~1µm2 in a homeotropic cell. Experimental points are shown by filled symbols. Red circles and arrows indicate the threshold power per area for the laser-induced transition.

Computer-simulated layered structure of the FCDs observed in the smectic phase of 8CB: (a) FCD with a hyperbola-ellipse pair of confocal defects; (b) the so-called toric FCD with the ellipse/hyperbola degenerated into the circle/straight line.

(a-c) CARS-PM images of a cholesteric finger structure obtained in a homeotropic cell with the E7-CB15 chiral nematic mixture; (d) reconstructed director structure in the finger’s vertical cross-section containing four nonsingular disclinations lines, of which two are λ1/2 defects of positive sign (red circles) and the other two λ-1/2 of negative sign (blue circles).

(a-c) CARS-PM images of an array of axially-symmetric cholesteric domains obtained in a homeotropic cell with the AMLC-0010+CB15 mixture; the red ellipsoids on the inset show areas in which molecular orientations closely match orientation of P̂CARS and correspond to high CARS intensity on the image. (d) schematics of the reconstructed n̂ -structure in the domain.